Could pesticides - their use and abuse - increase levels of cancer and birth defects? It is a question asked across the vast belt of Argentina where GM crops are grown. In Chaco, the Minister of Public Health wants an independent commission to investigate a growing health crisis.

On a hot, still day the buzz of a distant, small plane is just audible.

"They fumigate all around with airplanes", says Carlos Fria who is out shopping for fruit and vegetables in Avia Terai. This small rural community sits, slap bang in the middle of Chaco - the flat, sprawling, northern province that nudges the border with Paraguay.

Carlos lives near Avia Terai, and says agrochemicals are sprayed close to his home, contravening local laws.

"If the wind changes the agrochemicals come into the house. My uncle just died of cancer. My wife too passed away from cancer. Now many, many people are dying of cancer - it didn't used to be like that," he says.

Cancer is becoming more common everywhere. Could Carlos's experience just be part of life in the modern world? He doesn't think so.

"In my opinion, this has to do with the poison they put on the fields."

Chaco was once famous for its cotton. Beef cattle dotted the endless plains. Now soya dominates the landscape. Argentina has become one of the world's largest producers of GM soya, with some 20 million hectares under cultivation. It is the nation's leading export.

The biotechnology revolution in agriculture began to transform Argentina in the mid-1990s. And it is the agrochemicals that support the crops that worry people.

Carlos Fria lives in Avia Terai: "If the wind changes, the agrochemicals come into the house"

In local research published in 2012, of those surveyed in Avia Terai, nearly a third said someone in the family had cancer in the last 10 years. In comparison, in one of Chaco's ranching communities, just 3% of those asked said they had a relative with cancer.

One of the report's authors was Dr Maria del Carmen Seveso, who was in charge of intensive care in a hospital in the nearby town of Presidencia Roque Saenz Pena, before she retired. She says it reflects her own experience of an increase in patients with aggressive tumours. About a decade ago she noticed something else too - a rise in the number of pregnant women with eclampsia, a life-threatening condition.

"We saw 20 cases in five years. Then in 2006, we had 20 women just in that one year. In 2007 the number rose again - this was the year they fumigated the land with agrochemicals most intensively, and planted the most soya beans here. Then the doctor in charge of neo-natal services at the hospital gave me some data. It showed that around 3.5% of babies born at the hospital had birth defects - he said the more usual figure would be around 1%."

Dr Raul Horacio Lucero, head of molecular biology at the National University of the North East in Chaco, also collected data about disabled infants who began to be referred to him in the 1990s.

"I'm talking about all kinds of disability - some had missing limbs, others had aberrations of their sex organs," he says.

"I didn't see any alteration at a chromosomal level, so then I began questioning the mothers. What they all had in common was they had been exposed to agrochemicals."

But none of these observations amount to scientific proof.

Seveso's report, based on questionnaires and anecdotal evidence from local people like Carlos, is a long way from establishing a causal relationship between agrochemicals and cancer. And it isn't clear which pesticides women were exposed to.

There could be other possible reasons for the apparent rise in cases of eclampsia and birth deformities - perhaps more women with problematic pregnancies were being seen at the hospitals, or more sick babies were surviving. Health problems could also be explained, it's been suggested by scientists, by household chemicals, arsenic in the water, or heavy metals in the soil. No one knows.

Meanwhile, producers of agrochemicals say they are scientifically proven not to be carcinogenic, or to adversely affect reproduction or development.

But this has not halted the debate in Argentina.

The most commonly used herbicide in the country is glyphosate. It is the world's top-selling weed killer, and was developed in the 1970s by the multinational giant, Monsanto. Now it is produced by many companies, and approved as safe by regulatory agencies around the world, including in the US and Europe.

GM soya is resistant to glyphosate, so when it is applied to crops, it kills only the weeds and not the soya plants.

One of Argentina's most eminent scientists, Dr Andres Carrasco, from the University of Buenos Aires, exposed chicken and frog embryos to Monsanto's commercial formulation of glyphosate, called Roundup.

"We diluted the formulation 5,000 times, and some of the embryos died and some of them became malformed. I decided to inject pure glyphosate into embryos and we got the same result."

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The malformations affected the regions of the heart and the head. They were similar, says Dr Carrasco, to some of the defects that have shown up in children in the GM agricultural belt.

This was experimental, laboratory evidence:

"You cannot connect both things together because in order to know the cause of these malformations in children, you have to do an experiment with children and that's not possible", he continues. "But it's a red light to say we should look at the health problems in the territory where we are using these toxics."

"Dr Carrasco's study is refuted by a lot of other scientists as well as by us," says Luiz Beling, Monsanto's President for the southern Latin American region.

"We weren't surprised by the results given the methodology used. Embryos are not the best way to test the effectiveness of the products and the impact on human health. The best way to do it is to use live animals. We have a very robust data set that shows the product is very safe. There is no evidence that if properly used, glyphosate causes any harm to the health of a human being.

"I think the opportunity we have as an industry is - how do we make sure the regulation is in place and enforced so growers use the products the right way?"

Across the GM belt people complain that laws governing how close to communities crop spraying is allowed, are not respected. There are also reports of families using empty agrochemical containers to store water in the home.

Near Leones, deep in the soya country of the Province of Cordoba, Alejandro Ferrero's farm of 1,200 hectares has been in the family since 1921. Some 700 hectares are soya - a sea of golden beige, ready for harvest. Ferrero embraced GM technology right at the start of the biotech revolution, and for soya alone it has brought him a 40-50% rise in yield. He doesn't believe the claims of a connection between agrochemicals and ill-health.

Alejandro Ferrero has embraced GM technology on his farm in Cordoba

"I think there just aren't the serious scientific studies to prove it. There's a tendency to blame soya and GM crops for many bad things - it's political. But until I see serious scientific studies, I'll continue to use these chemicals. On the other hand, I'm conscious these are chemicals and we have to use them responsibly."

Ferrero says he has reduced the amount of agrochemicals he applies to his crops over the last 20 years. But across Argentina, their use has increased eight-fold since the early 1990s to more than 300 million litres a year.

What has happened is that weeds have become resistant, so chemicals are mixed, or they are used in higher concentrations. Or, farmers simply don't understand or follow the instructions properly.

One of the original arguments used to promote the introduction of genetically modified agriculture was that the burden of agrochemical use would fall. So given the levels of herbicides and pesticides being used in Argentina, is GM technology failing? Not according to Monsanto's Luiz Beling.

"Thirty-five million tons of grain were produced in Argentina in 1990, compared to about 100 million now. That's a huge growth - triple the amount of grain produced. Yes, the amount of chemicals used has gone up. But what nobody talks about is the level of toxicity. Prior to 1995 the level of toxicity of products was much higher. And with the advent of Roundup and glyphosate - a very safe product of low toxicity, the level of toxicity has gone down."

So what is the government's view on the debate in Argentina? The Ministries of Health, Agriculture and Technology all declined requests for an interview.

In Avia Terai, Viviana Perez hears the buzz of a light plane going over, on its way to fumigate the fields.

Sitting in her wheelchair next to Viviana in their small front garden, is her 14 year old daughter, Nadia.

"There is no medicine for my daughter. The doctors told me the cause of her illness was partly genetic, but also that agrochemicals were a big influence," says Viviana.

Nadia's illness is progressive.

"I would love my daughter to have the same ability as she did before - to be able to walk around and to talk to me."

There is no evidence to prove Nadia's illness developed as a result of her mother's exposure to crop spraying. In Chaco, the local Minister of Public Health, Antonio Morante, a medical doctor who was born in Avia Terai, is responding to the worries of locals like Viviana. He wants to commission an independent health study in the province.

"We want someone to come in from outside and carry out an exhaustive analysis of all these cases. As a doctor, as a minister and as a Chaqueno, I'm concerned young women are having recurrent miscarriages, that children are born with deformities, that there are many examples of cancer. But we also have many cases in areas where they don't use agrochemicals. So we must treat this with scientific rigour - that's what we're doing this year."

(Dr Andres Carrasco spoke to the BBC in Buenos Aires - his death was announced in Argentina on Saturday 10 May 2014).